ANSWER:

Saying the first inaugural takbeer is a pillar without which the prayer is invalid. You can say the takbeer again before joining the imam in his position but even if you didn’t, your prayer is valid.

when one enters the congregational salah late, does he say allahu Akbar when starting, and then say it again when going into the position as the imam, or does he just say it once and go into the position as the imam?